1. Field of the Invention
The present invention relates to semiconductor devices using a III-group element nitride (hereinafter, III-V nitride)for use in, for example, a reader of a high-density optical disk, including short-wavelength light emitting devices, such as a light emitting diode or a laser diode, light-receiving devices such as a photodiode, high-temperature semiconductor devices which can operate even in a high-temperature environment, and high-speed operation semiconductor devices.
2. Description of the Prior Art
A III-V nitride having a wurtzite structure is a direct transition type semiconductor having a wide forbidden band and, hence, has attracted attention as a material for a short-wavelength light emitting device or a light-receiving device and, further, a high-temperature semiconductor device and a high-speed semiconductor device. The III-V nitrides having the above wurtzite structure include gallium nitride (GaN), aluminum nitride (AlN), boron nitride (BN), indium nitride (InN), and single-crystalline alloys of these nitrides. The forbidden band width can be varied by varying the type of the material and the composition of the single-crystalline alloy.
In particular, gallium nitride and single-crystalline alloys thereof have been actively studied as the material of light emitting devices. In recent years, blue and bluish green LEDs (light emitting diodes) using a gallium nitride compound semiconductor have been put to practical use, and, in the next stage, practical use of short-wavelength LDs (laser diodes) in a reader for a high-density optical disk is expected in the art.
In the formation of semiconductor devices such as LEDs and LDs, a single crystal film of a semiconductor should be grown on a substrate of a single crystal of the same crystal structure. In the III-V nitrides, however, a single crystal having a size large enough to be used as the substrate could have not been synthesized, and, hence, hetero-epitaxial growth wherein a single crystal film of a III-V nitride is grown on a dissimilar substrate has been used in the art. Substrates known to be usable for the hetero-epitaxial growth include the (0001) plane (plane C) and the (11-20) plane (plane A) of a single crystal of sapphire, the (111) plane of a single crystal of silicon, the (0001) plane of a single crystal of 6H-SiC, and the (111) plane of a single crystal of MgAl.sub.2 O.sub.4.
The single crystal substrate of sapphire among these substrates permits the growth of a single crystal film of a gallium nitride compound semiconductor having good crystallinity through a low-temperature growth buffer layer and, in addition, is relatively inexpensive. Therefore, it is most commonly used in blue and bluish green LEDs utilizing the above gallium nitride compound semiconductor.
Particularly regarding LDs, however, those in an infrared region using a GaAs compound semiconductor or an InP compound semiconductor have been put to practical use, but blue-emitting LDs using a III-V nitride are still under study and there are some reports thereon. Examples of such reports include Jpn. J. Appl. Phys., Vol. 35 (1996), pp. L74-L76, Jpn. J. Appl. Phys., Vol. 35 (1996), pp. L217-L220, and Appl. Phys. Lett., Vol. 68, No. 15 (1996), pp. 2105-2107. All the LDs cited in these reports are of a pulse oscillation type.
Appl. Phys. Lett., Vol. 69, No. 26 (1996), pp. 4056-4058 reports a continuous oscillation type LD. This LD has a short service life due to a temperature rise during continuous oscillation. This is because, since the single crystal substrate is not constituted of a III-V nitride, the differences in the lattice constant and in the coefficient of thermal expansion between the substrate and the overlying single crystal film of a III-V nitride are so large that it is difficult to synthesize a single crystal film of a III-V nitride having good crystallinity, which results in inclusion of many defects such as dislocation in the single crystal film.
Thus, high-temperature semiconductor devices and high-speed semiconductor devices using III-V nitrides have not been yet put to practical use, although application of III-V nitrides as a light emitting device has been partly realized. This also is because many defects such as dislocation are included in the single crystal film of a III-V nitride. The dislocation is propagated under high temperature conditions, leading to remarkably deteriorated performance, which results in a shortened service life. Further, the dislocation lowers the mobility of the carrier unfavorably, making it impossible to realize high-speed operation.
For this reason, improving the film quality by interposing a low-temperature buffer layer has been attempted. At the present time, however, the improvement is unsatisfactory. In addition, the cleavage of the single crystal substrate used is difficult, posing problems that the flatness of the mirror facet for a laser cavity cannot be ensured in the formation of LD and, in addition, the step of forming the mirror facet for a laser cavity is complicate.
In order to solve the above problems, an attempt to utilize a single crystal of gallium nitride as a III-V nitride for a single crystal substrate has been reported in Jpn. J. Appl. Phys., Vol. 35 (1996), pp. L77-L79. The size of the single crystal of gallium nitride which can be currently synthesized is small and up to about 2 mm square, making it difficult to put the single crystal to practical use in semiconductor devices such as LDs which can be continuously oscillated.
Further, heat dissipation should be improved because light emitting devices, particularly LDs, have a high calorific value at the time of light emission, high-speed semiconductor devices have a high calorific value at the time of high-speed operation, and, further, high-temperature semiconductor devices also need the prevention of deterioration thereof. For these reasons, the use of substrates having high heat conductivity is desired. In this connection, however, it should be noted that all of substrates made of sapphire, silicon, MgAl.sub.2 O.sub.4, and the like, which permit the crystal growth of a necessary single crystal film, have low heat conductivity. Therefore, in devices where high output and temperature stability are required, these substrates are used in the form bonded to a heat sink material. Jpn. J. Appl. Phys., Vol. 34 (1995), pp. L1517-1519 proposes a device comprising a single crystal substrate of sapphire and a GaN layer provided on the substrate through an AlN film. In this device, however, the above problem of heat dissipation remains unsolved because the single crystal of sapphire is used as the substrate.
An example of the growth of a semiconductor material on a substrate having high heat conductivity is described in Japanese Patent Laid-Open No. 42813/1989 wherein a single crystal film of a semiconductor is provided on a single crystal substrate of diamond. The substrate in this example is a thin-film single crystal substrate comprising a single crystal substrate of diamond bearing at least one single crystal layer formed of at least one material selected from among gallium nitride, indium nitride, aluminum nitride and the like. This example is aimed at providing a substrate having high heat conductivity, low coefficient of thermal expansion, and excellent heat resistance and environmental resistance. This single crystal substrate of diamond also is unsatisfactory in matching in lattice constant and coefficient of thermal expansion with the single crystal of a III-V nitride.
As described above, substrate materials, such as sapphire, silicon, SiC, and MgAl.sub.2 O.sub.4, which have hitherto been used for the growth of a single crystal film of a III-V nitride are unsatisfactory in matching in lattice constant and coefficient of thermal expansion with the III-V nitride. This makes it difficult to synthesize a single crystal film of a III-V nitride having good crystallinity on these substrates, and, at the present time, the interposition of a low-temperature growth buffer layer between the single crystal film and the substrate is necessary and indispensable.
For all the conventional single crystal substrates except for SiC, the cleavage for the formation of a mirror facet for a laser cavity in LDs was impossible. Although an attempt has been made to form a mirror facet by etching, this method cannot provide a satisfactorily even laser cavity and, at the same time, is disadvantageous in that the step of forming a mirror facet is complicate. On the other hand, the use of SiC as the substrate results in poor light emitting properties. Although the reason for the poor light emitting properties has not been elucidated yet, it is believed to reside in the stress derived from the difference in the coefficient of thermal expansion or diffusion of the SiC component in the GaN film. For the sapphire substrate which has been most commonly used in the art, the heat conductivity is so low that, for use in light emitting devices, the substrate should be bonded to a heat sink. Further, in this case, since sapphire is not electrically conductive, it is difficult to handle an electrode in the bonding to the heat sink.
As described in Jpn. J. Appl. Phys., Vol. 35 (1996), pp. L77-L79, the use of a single crystal substrate of gallium nitride enables the synthesis of a high-quality film of a III-V nitride. At the present time, the formation of a large single crystal of gallium nitride to be used as the substrate is difficult, and, in addition, the thermal conductivity is poor, offering poor heat dissipation. In the technique disclosed in the Japanese Patent Laid-Open No. 42813/1989, a single crystal substrate of diamond having high thermal conductivity is used for heat dissipation purposes. This method is, however, disadvantageous in that this substrate is unsatisfactory in matching in lattice constant and coefficient of thermal expansion with the single crystal of a III-V nitride.